This invention relates to variable capacity compressors for compressing refrigerant gas circulating in an air conditioner adapted especially for use in automotive vehicles, and more particularly to improvements in or to a capacity control system of a compressor of this kind, employing an electromagnetic valve which is opened and closed to control the delivery quantity or capacity of the compressor.
Conventionally, a capacity control system of a variable capacity vane compressor of this kind has been proposed, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 2-64779 (corresponding to U.S. Pat. No. 5,056,990 to Nakajima), which comprises a control element disposed to rotate between the minimum capacity position and the maximum capacity position for controlling the timing of start of compression, a low-pressure chamber which is defined on one side of a pressure-receiving protuberance of the control element and into which is introduced suction pressure Ps as low pressure, a high-pressure chamber defind on the other side of the pressure-receiving protuberance and into which is introduced discharge pressure Pd as high pressure via a restriction passage to create control pressure Pc therein, the control element being rotated in response to the difference between the sum of the suction pressure Ps introduced into the low-pressure chamber and the urging force of urging means, and the control pressure Pc, and an electromagnetic valve for opening and closing a passageway which communicates between the high-pressure chamber and a suction chamber, wherein the opening and closing of the passageway by the electromagnetic valve is controlled by a pulse signal to control the flow rate of refrigerant gas leaking from the high-pressure chamber into the suction chamber through the passageway to vary the control pressure within the high-pressure chamber such that the control element is rotated in accordance with variation in the control pressure, to thereby control the capacity of the compressor in a continuous manner.
According to this conventional capacity control system, the passageway is opened when a pulse signal supplied to the solenoid of the electromagnetic valve is on, while it is closed when the pulse signal is off. The duty ratio of the pulse signal is controlled in accordance with thermal load on the compressor, whereby the leak amount per unit time of the refrigerant gas is controlled to thereby control the angular position of the control element.
In this prior art, when the angular position of the control element is changed, the duty ratio is maintained at a constant value during a time period between the start of rotation of the control element from a stationary state and stoppage of rotation of same in a desired position. However, this system has the drawback that it is incapable of quickly starting rotation of the control element from a stationary state. More specifically, no countermeasure has been taken against the frictional force (static frictional force) between a seal member mounted on the periphery of the control member and opposed walls of the compressor, and the hysteresis characteristic of the seal member, so that the capacity control system suffers from poor responsiveness and cannot effect smooth and delicate control of the delivery quantity or capacity of the compressor.
In the meanwhile, an electromagnetic valve for use in a capacity control system of this kind has been proposed e.g. by Japanese Utility Model Application No. 2-49277 (corresponding to Japanese Published Utility Model Application (Kokai) No. 4-8790), which comprises a spool valve having a spool valve body which is displaceable between an open position, in which a high-pressure chamber is communicated with a suction chamber, and a closed position, in which the communication between the chambers is cut off, a spring urging the spool valve body toward the closed position, and an electromagnetic actuator which generates an electromagnetic force in response to an external control signal for magnetically attracting the spool valve body toward the open position against the force of the spring.
However, according to this proposed electromagnetic valve, the spool valve body allows control pressure to leak into the suction chamber. This structure requires high airtightness between the spool valve body and its associated parts for prevention of undesired leakage of control pressure through clearances between the spool valve body and its associated parts, which necessitates the use of a spool valve in which the spool valve body has a long stroke. This results in an increased size of the electromagnetic actuator, and hence in an increased size of the compressor. Further, this capacity control system has the drawbacks of increased electric power consumption and poor responsiveness.